Subscriber line interface circuit

ABSTRACT

In a subscriber line interface circuit, a device applies to a line, associated with the line interface circuit, a substantially constant line current of a first predetermined value for line voltages up to a first voltage having an absolute value which, by a predetermined amount, is lower than the supply voltage of the line interface circuit. For line voltages between the first voltage and a second voltage having an absolute value which, by a predetermined amount, is higher than the first voltage, a line current which is inversely proportional to the line voltage and of a value between said first predetermined value and a second predetermined, lower value is applied to the line. For line voltages between the second voltage and a third voltage having an absolute value which, by a predetermined amount, is higher than the second voltage, a substantially constant line current of the second predetermined value is applied to the line. For line currents of lower value than said second predetermined value, the line voltage is maintained substantially constant at the third voltage.

This application is a continuation of U.S. patent application No.08/961,017, entitled “Subscriber Line Interface Circuit” filed on Apr.30, 1997,” U.S. Pat. No. 6,122,367.

TECHNICAL FIELD

The invention relates to a subscriber line interface circuit.

BACKGROUND

Today's subscriber line interface circuits should imitate thetraditional way of feeding a telephone line from an exchange battery viaa feeding resistance. Thus, this feeding resistance determines thedependence of the line current on the line voltage, i.e. the line feedcharacteristic of the line interface circuit.

However, different countries require different feeding resistances,which results in that today's line interface circuits are not generallyusable but have to be adapted to the requirements specified in therespective country.

For a short-circuited line and for low-resistance line loads, the linecurrent will be high which causes a high power dissipation in thefeeding resistance. To avoid this, it is known to limit the maximumvalue of the line current. Also, this maximum value differs from countryto country and, consequently, today's line interface circuits have to beadapted also in this respect to the requirements specified in therespective country. However, in today's line interface circuits, it isnot possible to arbitrarily limit the line current to desired values ina simple way.

When the line is open, i.e. with the handset on-hook, today's lineinterface circuits have the disadvantage that the line voltage can belower than the expected, ideal line voltage for a given feeding voltage.This can depend e.g. on the fact that the associated line may have aleakage resistance or that the device, e.g. a telephone set, connectedto the line draws current from the line for such reason. The value ofthis open-line voltage is, however, very important for some units, e.g.so-called MTUs (Maintenance and Test Units) and certain facsimileapparatuses.

Moreover, today's line interface circuits are not adapted to adapt theirline feed characteristic to possible feeding voltage or supply voltagevariations.

SUMMARY

The object of the invention is to eliminate the above disadvantages ofsubscriber line interface circuits known so far.

This is attained by means of the subscriber line interface circuitaccording to the invention mainly in that it comprises means adapted toa line, associated with the line interface circuit, an essentiallyconstant line current of a first predetermined value for line voltagesup to a first voltage having an absolute value which, by a predeterminedamount, is lower than the supply voltage of the line interface circuit,means adapted to apply to the line, a line current which is inverselyproportional to the line voltage and of a value between said firstpredetermined value and a second predetermined, lower value for linevoltage between the first voltage and a second voltage having anabsolute value which, by a predetermined amount, is higher than thefirst voltage, means adapted to apply to the line, an essentiallyconstant line current of the second predetermined value for linevoltages between the second voltage and a third voltage having anabsolute value which, by a predetermined amount, is higher than thesecond voltage, and means adapted to maintain the line voltageessentially constant at the third voltage for line currents of lowervalue than said second predetermined value.

Alternatively, the line interface circuit according to the inventioncomprises means adapted to apply to a line, associated with the lineinterface circuit, an essentially constant line current of a firstpredetermined value for line voltages up to a first voltage having anabsolute value which, by a predetermined amount, is lower than thesupply voltage of the line interface circuit, means adapted to apply tothe line, a line current which is inversely proportional to the linevoltage and of a value between said first predetermined value and asecond predetermined, lower value for line voltages between the firstvoltage and a second voltage having an absolute value which, by apredetermined amount, is higher than the first voltage, and meansadapted to maintain the line voltage essentially constant at the secondvoltage for line currents of lower value than said second predeterminedvalue.

Hereby, the subscriber line interface circuit according to the inventionwill be insensitive to supply voltage variations as well as to leakagecurrents on the line, and will also be easily adaptable to requirementsin different countries.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described more in detail below with reference tothe appended drawing, on which

FIG. 1 schematically shows a first embodiment of a subscriber lineinterface circuit according to the invention,

FIG. 2 shows the line feed characteristic for the line interface circuitin FIG. 1,

FIG. 3 schematically shows a second embodiment of the line interfacecircuit according to the invention, and

FIG. 4 shows the line feed characteristic for the line interface circuitin FIG. 3.

DETAILED DESCRIPTION

FIG. 1 schematically shows a first embodiment of a subscriber lineinterface circuit according to the invention. In a manner known per se,the line interface circuit is connected to the A-wire 1 and B-wire 2 ofa telephone line via a respective output amplifier (not shown) in adriving stage 3. The driving stage 3 is connected between ground and asupply voltage VBAT which normally is supplied by a battery (not shown).

As shown in FIG. 1, the telephone line is terminated by means of aresistor RL which represents the sum of the resistance of the line andthe resistance of a device connected to the line, e.g. a telephone set(not shown).

The control input terminal of the driving stage 3 is connected to thecollector of a transistor Q1. The base of the transistor Q1 isconnected, on the one hand, to the output terminal of a transconductanceamplifier 4 having a transconductance gm and, on the other hand, to thesupply voltage VBAT via a current a current generator 11.

The “+”-input terminal of the transconductance amplifier 4 (the upperinput terminal in FIG. 1) is connected to the B-wire 2, while its“−”-input terminal (the lower input terminal in FIG. 1) is connected, onthe one hand, to the supply voltage VBAT via a resistor R and, on theother hand, to the emitter of a transistor Q2 via two series-connecteddiodes D1, D2. The voltage across the resistance R is denoted UR, whilethe voltage across the diodes D1, D2 is denoted U1.

The base of the transistor Q2 is connected to the interconnection pointbetween the emitter of the transistor Q1, the base of a transistor Q3and the collector of a transistor Q4, while the collector of thetransistor Q2 is connected to ground. The voltage between thisinterconnection point and the supply voltage VBAT is denoted U2.

The collector of the transistor Q3 is connected to the supply voltageVBAT, while its emitter is connected, on the one hand, to ground viaprogrammable current generator I2 and, on the other hand, to the anodeof a diode D3 whose cathode is connected on the one hand, to the anodeof a diode D4 and, on the other hand, to the base of the transistor Q4.

The emitter of the transistor Q4 is interconnected with the cathode ofthe diode D4 and the interconnection point is connected to the supplyvoltage VBAT via a settable resistor R1.

The transistor Q4 and the diodes D3 and D4 together form a currentmirror to mirror the current flowing through the diodes D3, D4 to thecollector of the transistor Q4. The sum of the current through thediodes D3, D4 and the collector current of the transistor Q4 will flowthrough the resistor R1. The voltage appearing across the resistor R1 isdenoted UR1.

In a manner not shown, but known per se, the voltage between the A-wire1 and ground is maintained equal to the voltage between the B-wire 2 andthe “−”-input terminal of the transconductance amplifier 4. These twoequal voltages, usually called “guardband”, are denoted UG. Theguardbands are there to enable speech signals and voice frequencysignalling on the line also when the handset is on-hook, and aredetermined in the embodiment shown by the current of the currentgenerator I1 and the transconductance gm of the transconductanceamplifier 4 in such a manner that UG=I1/gm.

In accordance with the invention, the line interface circuit shown inFIG. 1 is adapted to bring about the line feed characteristic shown inFIG. 2 when the line load RL varies from a short-circuit to an openline. As apparent from FIG. 2, the line interface circuit applies anessentially constant line current IL of a predetermined value IL1 to theassociated line for line voltages UL up to a voltage UL1. The absolutevalue of the voltage UL1 is lower than the supply voltage VBAT of theline interface circuit by a predetermined amount. For line voltagesbetween the voltage UL1 and a voltage UL2 having an absolute value whichis higher than the voltage UL1 by a predetermined amount, the lineinterface circuit according to FIG. 1 applies a line current ILinversely proportional to the line voltage UL and of a value between IL1and a predetermined, lower value IL2 to the line. For line currents ILlower than IL2, the line interface circuit maintains the line voltage ULsubstantially constant at the voltage UL2.

To accomplish this, the programmable current generator I2 is programmedto apply a line current IL of the desired, substantially constant valueIL1 according to FIG. 2 to the line 1, 2. The current from the currentgenerator I2 is mirrored to the collector of the transistor Q4 andapplied to the control input terminal of the driving circuit 3 via thetransistor Q1. On the basis of the current appearing of the controlinput terminal, the driving circuit 3 applies a corresponding linecurrent of the value IL1 to the line 1, 2. Under these conditions, thetransistor Q3 is cut off.

Thus, the line voltage UL will be equal to IL1×RL. Load resistances RLof different values will, therefore, give line voltages UL of differentvalues as long as IL=IL1.

Usually, the line current IL1 is so chosen that it corresponds to thecurrent at which the telephone set in the application in questionpractically ceases to the compensate for line length dependingattenuation on the line.

According to the invention IL=IL1 for line voltages between 0V, i.e.short-circuited line, and the line voltage UL1. This line voltage hasbeen set so that |UL1|=|VBAT·2UG+U1−UR1|, i.e. the absolute value of theline voltage UL1 is lower than the supply voltage VBAT by apredetermined amount. UG and U1 are constant and preset, and UR1 isconstant when |UL|<|UL1|.

When the line voltage UL becomes higher than UL1 due to the fact thatthe line load RL has increased, e.g. in that a device having a higherresistance has been connected to the line or that a longer line has beenconnected to the line interface circuit, the transistor Q3 starts toconduct. Hereby, the current through the diodes D3 and D4 will be lowerand, thereby, also the current through the resistor R1 which causes thevoltage UL1 to become lower. When |UL|<|UL1|, UR1=UR+U1, the voltage U1being constant and the voltage UR being inversely proportional to theline voltage UL. Also, the collector current of the transistor Q4 andthe current through the transistor Q1 will be lower. Thus, the controlcurrent of the driving stage 3 will be lower which in its turn causesthe line current IL to be lower than IL1. The line current IL will,thus, be inversely proportional to the line voltage UL.

If the current through R1 becomes still lower, finally UR=0 when UR1=U1.

Then, the feedback loop to the transconductance amplifier 4, comprisingthe transistor Q2 and the diodes D1, D2, will be broken which causes theamplification of the transconductance amplifier 4 to suddenly becometremendously high. Therefore, a small change of the line voltage UL willresult in a very big change of the voltage UR1. Since the voltage UR1determines the line current IL, a big line current change is obtainedfor a small line voltage change. The value of the line current IL2 whenUR1=U1, is determined by the voltage U1 across the diodes D1, D2 and bythe resistance of the resistor R1. If the resistance of the resistor R1has been chosen in response to a requirement for a certain inclinationof the line feed characteristic, the current IL2 can, thus, be set by asuitable choice of the voltage U1. The value of the current IL2 ischosen in view of the leakage current that can be expected in dependenceon the value of the line leakage resistance.

Hereby, |UL2|=|VBAT=2UG|. Thus, the absolute value of this open-linevoltage is lower than the supply voltage VBAT by a predetermined amountbut higher than the line voltage value UL1 by a predetermined amount.

As mentioned in the introductory portion, for their operation, certaindevices are dependent on a certain minimum line voltage when the handsetis on-hook, i.e. a certain open-line voltage. By choosing this open-linevoltage, |UL2|=|VBAT−2UG|, in accordance with the invention, thisvoltage is maintained constant also if current is drawn from the line bya leakage resistance or by a device which draws current when the handsetis on-hook. To keep the power losses low in the line interface circuitsand at the same time keep the costs low for the exchange batteries, itis of course desirable that the open-line voltage is reached at abattery voltage which is as low as possible.

The appearance of the line feed characteristic between the pointsIL1/UL1 and IL2/UL2 in FIG. 2, i.e. its inclination, has to fulfill therequirements specified by the respective country as also mentioned inthe introductory portion. According to the invention, the inclination ofthe line feed characteristic is changed simply by changing theresistance of the resistor R1. Hereby, it will be very easy to adapt theline interface circuit according to the invention to the requirementsspecified by the respective country.

FIG. 3 shows a second embodiment of the subscriber line interfacecircuit according to the invention, which to a great extent correspondsto the embodiment according to FIG. 1. Elements in FIG. 3 which areidentical with elements in FIG. 1 have been provided with identicalreference characters and will not be described in any greater detail inconnection with FIG. 3.

According to the invention, the line interface circuit according to FIG.3 is adapted to generate the line feed characteristic shown in FIG. 4,which differs from the characteristic shown in FIG. 2 merely in that theline current is maintained essentially constant at the value IL2 forline voltages between a voltage UL2′ and a voltage UL3.

Thus, the line interface circuit in FIG. 3 is adapted to apply anessentially constant line current IL of the value IL1 to the associatedline for line voltages UL up to the voltage UL1 which in the same manneras in FIG. 2 is of an absolute value which is lower than the supplyvoltage VBAT of the line interface circuit by a predetermined amount.For line voltages between the voltage UL1 and the voltage UL2′ which isof an absolute value which is higher than the voltage UL1 by apredetermined amount, the line interface circuit according to FIG. 1applies, in the same manner as the line interface circuit according toFIG. 1, a line current IL which is inversely proportional to the linevoltage UL and of a value between IL1 and IL2 to the line. In contrastto the line interface circuit according to FIG. 1, the line interfacecircuit according to FIG. 3 is adapted a substantially constant linecircuit IL of the value IL2 to the line for line voltage UL between thevoltage UL2′ and the voltage UL3 which is of an absolute value which ishigher than the voltage UL2′ by a predetermined amount. Then, the lineinterface circuit according to FIG. 3 is adapted to maintain the linevoltage UL substantially constant at the voltage UL3 for line currentsof lower value than IL2.

In the embodiment according to FIG. 3, the diodes D1 and D2, and theresistor R have been left out. Instead, a resistor R3 is connectedbetween the emitter of the transistor Q2 and the collector of thetransistor Q5 and the interconnection point between the resistor R3 andthe collector of the transistor Q5 is connected to the “−”-inputterminal of the transconductance amplifier 4. The emitter of thetransistor Q5 is connected to the supply voltage VBAT. A currentgenerator I3 is connected between the supply voltage VBAT and the baseof the transistor Q5, while a current generator I4 is connected betweenground and the base of the transistor Q5. As to the rest, the circuitshown in FIG. 3 corresponds to the circuit shown in FIG. 1.

The current generator I3 is adapted to output a current proportional tothe line current IL, i.e. I3=k×IL, while the current generator I4 isadapted to output a selectable, constant current I4.

The current I4 is selected in such a manner that I3=I4 when IL=IL2 inaccordance with FIG. 4. Hereby, the current IL2 will be independent ofthe value chosen for the resistor R1.

The transistor Q5 is cut off for line currents IL>IL2, i.e. for I3>I4,and, thus, does not have any function under these conditions.

However, the transistor Q5 starts to conduct for line currents IL<IL2.The transistor Q5 becomes saturated when the line voltage UL3 isreached. Also, |UL3|=|VBAT=2UG|.

Thereby, the feedback is interrupted to the “−”-input of thetransconductance amplifier 4. This increases the amplification in thetransconductance amplifier 4 substantially. Even at a small variation ofthe line voltage UL, a large variation of the line current IL isobtained.

As indicated in the introductory portion above, a problem with today'sline interface circuits is that they are not adapted to adapt their linefeed characteristic to possible supply voltage variations.

As should be apparent from the above, this problem is solved, inaccordance with the invention, by tying the voltages UL1, UL2 and UL2′,respectively, and UL3 to the supply voltage VBAT. Hereby, uponvariations in the supply voltage VBAT, the voltages UL1, UL2 and UL2′,respectively, and UL3 will be displaced along the UL axis in FIGS. 2 and4, respectively, while maintaining the mutual “distances” to the supplyvoltage VBAT.

What is claimed is:
 1. A subscriber line interface circuit, comprising:means adapted to apply to a line, associated with the line interfacecircuit, a substantially constant line current, IL₁ for line voltages upto a first voltage, UL₁, wherein the absolute value of UL₁ is lower thana supply voltage, V_(BAT), by a fixed amount; means adapted to apply tothe line, a line current which is inversely proportional to the linevoltage and a value between IL₁ and a second current IL₂ for linevoltages between the first voltage, UL₁, and a second voltage, UL₂,wherein the absolute value of UL₂ is higher than UL₁ by a fixed amount;and means adapted to maintain the line voltage substantially constantfor line currents less than IL₂, such that the line interface circuit isself-adjusting with respect to supply voltage variations.
 2. The lineinterface circuit as claimed in claim 1, further comprising meansadapted to set the first predetermined value of the line current to adesired value.
 3. The line interface circuit as claimed in claim 1,further comprising means adapted to set the constant of proportionalitybetween a line voltage and the line current to a desired value.
 4. Asubscriber line interface circuit to claim 1, wherein the means adaptedto apply to a line, associated with the line interface circuit, asubstantially constant line current, IL₁, for line voltages up to afirst voltage, UL₁, wherein the absolute value of UL₁ is lower than asupply voltage, V_(BAT), by a fixed amount comprises a programmablecurrent generator programmed to generate a line current IL correspondingto the substantially constant line current IL_(1.)
 5. A subscriber lineinterface circuit according to claim 4, wherein the line current ILgenerated by the programmable current generator is applied as a controlinput current signal to a driving circuit via a first trnasistor.
 6. Asubscriber line interface circuit according to claim 1, wherein themeans adapted to apply to the line, a line current which is inverselyproportional to the line voltage and of a value between IL₁ and a secondcurrent IL₂ for line voltages between the first voltage, UL₁, and secondvoltage, UL₂, wherein the absolute value of UL₂ is higher then UL₁ by afixed amount comprises a circuit adapted to reduce the control inputcurrent signal applied to the driving circuit via a first trnasistor inresponse to increases in response to an increase in the line volatage.7. A subscriber line interface circuit according to claim 6, wherein thecircuit adapted to reduce the control input current signal applied tothe driving circuit in response to an increase in the line voltagecomprises a second transistor disposed between the first collector ofthe transistor and the control input of the driving circuit.